One component of a zero discharge pulp mill, such as described in the parent application, is a system for the manufacture of chlorine dioxide for use in the bleach plant of the pulp mill. It is highly desirable to be able to generate all of the chlorine dioxide on site, and in a manner recovering chemicals from the pulp production.
In the parent application, chlorine dioxide generation is accomplished by concentrating the bleach plant effluent, incinerating it, and then subjecting it to a leaching stage in which the sodium salts are leached prior to crystal washing. Sodium chloride, which is removed during crystal washing, is directed to chlorine dioxide production while part of the leachate is directed back to the recovery boiler of a chemical recovery loop of the pulp mill.
According to the present invention, another method of producing chlorine dioxide, suitable for use in the zero discharge pulp mill of the parent application, is provided, which avoids the leaching and crystal washing stages. In the various aspects of the present invention, after concentration and incineration of bleach plant effluents, the ash produced may be fed directly to chlorine dioxide production, or after storage to chlorate manufacture, or a part to each. In a chlorate manufacture stage the ash is chemically reacted to produce chlorate which is then subsequently used in chlorine dioxide manufacture.
According to one aspect of the present invention a method of producing chlorine dioxide for a cellulose pulp mill having a bleach plant, is provided. The method comprises the steps of automatically, continuously and sequentially: (a) Concentrating liquid effluents from the bleach plant to a concentration level high enough for incineration. (b) Incinerating the concentrated bleach plant effluents to produce an ash. (c) Chemically reacting at least a part of the ash from step (b) to produce chlorate. And, (d) using the chlorate produced in step (c) in the manufacture of chlorine dioxide, which chlorine dioxide is used in the bleach plant to bleach pulp. Step (c) is practiced by purifying the ash to produce sodium chloride, and reacting sodium chloride from the ash with oxygen and external energy to produce sodium chlorate. This also produces sulfates and heavy metal hydroxides. The sulfates are used to produce acid and caustic, which are used elsewhere in the mill. Part of the ash from step (b) can be used directly in chlorine dioxide manufacture. In the chlorine dioxide manufacture caustic and acid also can be produced, as are metal hydroxides (which are disposed of).
According to another aspect of the present invention, a method of producing chlorine dioxide for a cellulose pulp mill having a bleach plant is provided. The method comprises the steps of automatically, continuously and sequentially: (a) Concentrating liquid effluents from the bleach plant to a concentration level high enough for incineration. (b) Incinerating the concentrated bleach plant effluents to produce an ash. And, (c) using at least a part of the ash directly in the manufacture of chlorine dioxide, which chlorine dioxide in turn is used in the bleach plant to bleach pulp.
According to another aspect of the present invention, a method of minimizing effluents from a cellulose pulp mill having a bleach plant and a recovery boiler and chemical recovery loop, is provided. The method comprises the steps of continuously: (a) Concentrating liquid effluents from the bleach plant to a concentration level high enough for incineration. (b) Incinerating the concentrated bleach plant effluents to produce an ash containing sodium carbonate, sodium sulfate, and sodium chloride. (c) Chemically reacting at least a part of the ash to produce chlorate and sodium salts. (d) Feeding at least a substantial portion of the sodium salts to the chemical recovery loop of the recovery boiler. And, (e) using the chlorate produced in step (c) in the manufacture of chlorine dioxide, which is used in the bleach plant.
It is the primary object of the present invention to provide a method for the effective production of chlorine dioxide, preferably all the chlorine dioxide needs for a bleach plant of a pulp mill, from bleach plant effluents. This and other objects of the invention will become clear from a detailed description of the drawing and from the appended claims.